The High Court has directed Garda Commissioner Drew Harris to disclose the names and addresses of gardaí against whom he believes there is evidence of involvement in a smear campaign against a former colleague.

Ex-garda Keith Blythe had brought legal proceedings seeking to find out the identities of former colleagues who allegedly defamed him in messages circulated via WhatsApp and Facebook.

Late last night Mr Justice Richard Humphreys directed the commissioner to disclose to Mr Blythe's legal team, led by Paul O'Higgins SC, the names and addresses of any person in relation to whom the commissioner considers there is prima facie evidence of involvement in the publication of defamatory information against Mr Blythe.

Mr O'Higgins told the court the alleged defamatory material had been spread on the internet in September 2018.

Judge Humphreys heard the alleged defamatory information had been posted in an apparent effort to smear Mr Blythe after he had issued High Court proceedings that had the potential of temporarily blocking the promotion of hundreds of gardaí.

The judge directed that the information be provided to Mr Blythe's legal team before 1pm today, so that defamatory proceedings may be issued against the proposed defendants prior to the Friday expiry of a time limit in which to launch defamation cases against them.

Judge Humphreys told Conor Power SC, who appeared with barrister James Geoghegan and solicitor Jevon Alcock for the Garda Commissioner, that he would not grant a stay on his order to facilitate an appeal on the grounds it might nullify the effect of the order he had just made.

The judge granted what is known as a Norwich Pharmacal Order requiring a third party, in this case the commissioner, to disclose the identity of an unknown wrongdoer or wrongdoers supposedly mixed up in the publications.

Mr O'Higgins told the court that the circulation of the defamatory remarks had caused Mr Blythe considerable distress.

Mr Power had strenuously opposed the granting of the order.

In September last year Mr Blythe issued High Court proceedings aimed at halting a Garda sergeant promotion competition until an internal appeal he had lodged had been fully investigated.

Within days of his proceedings being served he had been made aware by 15 colleagues about the remarks that had been published about him.

Mr Blythe's proceedings had the potential to temporarily prevent the promotion of 410 candidates selected from more than 1,400 who had sought promotion to the rank of sergeant.

His proceedings were struck out after the court had been told the matter had been resolved.

Mr Blythe, on being given the names and addresses of those allegedly responsible, intends immediately issuing defamation claims against them.